Implants are used very often in surgical, orthopedic, dental, and other related applications, including tissue engineering. One important issue with implants is that due to biomechanical and physiologic requirements, an implant material should have a certain mechanical strength or elasticity to be incorporated into the target tissue and anatomic region. Also, degradability or possibly even incorporating pharmacologically or therapeutically active agents is also desirable.
Several different materials for implants have been used, including metal. Metallic implant materials are usually favorable in terms of toughness, ductility, and fatigue resistance. On the other hand, they are often stiffer than natural bone, resulting in stress shielding. The phenomenon of stress shielding is based on the effect that the implant material bears more of the mechanical load if it is stiffer than the surrounding tissue. This results in a “shielding” of the natural bone tissue from the mechanical load triggering the resorption processes of bone.
Patients with orthopedic fractures or deformities are sometimes treated with surgically implanted metallic materials. The most common metallic materials used in fracture fixation or total joint devices are medical-grade non-degradable metals, such as stainless steel, titanium, and cobalt-chromium-based alloys. Existing permanent metallic implants for fracture fixation and total hip replacement can often cause stress shielding effects due to the mismatch of the mechanical properties between these metallic implants and natural bone. That is, due to a mismatch of the mechanical properties between these metallic implants and natural bone, the major obstacle in using these non-degradable permanent metal implants is that they may cause stress shielding effects, thereby leading to bone loss around the implant. Patients who undergo orthopedic procedures such as fracture fixation, often undergo a second surgery after treatment is done in order to avoid this post-operative complication.
Polymeric implants are sometimes used as an alternative to metallic implants. However, existing polymeric implants often do not have appropriate mechanical strength to withstand load-bearing conditions.
Biodegradable metallic implants such as magnesium alloys have sometimes been used. However, in addition to a mismatch in mechanical properties and poor biocompatibility, magnesium alloys exhibit problems with a rapid degradation rate and hydrogen gas accumulation upon implantation. Rapid corrosion results in the release of a large amount of magnesium ions together with a large volume of hydrogen gas generated. As shown in Reactions (1)-(3) below, if an increase in corrosion rate leads to an increase in magnesium ions. Accordingly, magnesium hydroxide is formed, and hydrogen gas is generated. The human body itself is able to absorb a small amount of hydrogen gas.
An alloying modification method is sometimes applied to attempt to improve the corrosion resistance of magnesium alloys. Although alloying can improve the corrosion resistance of magnesium alloy, this technique may introduce biological toxicity due to the use of rare earth metals, such as cerium and yttrium. Additionally, the compatibility with living cells remains a problem. The mechanical properties of magnesium alloy are closer to human natural bone than those of other metallic materials such as titanium alloys and stainless steel. However, there is still a discrepancy between magnesium alloy and natural bone in terms of bulk mechanical properties.H2O→H++OH−  Reaction (1):2Mg→Mg2++2e−  Reaction (2):Mg+2H2O→Mg(OH)2+H2  Reaction (3):
Each of the existing materials used for orthopedic implants exhibits potentially harmful problems. Thus, there exists a need in the art for an improved material that can be used as an implant in orthopedic and other medical applications, as well as in other non-medical applications where a durable substitute for traditional metals or plastics is desired.